Preliminary Analysis of the Borehole Strainmeter Data in Eastern Taiwan
Date Issued
2012
Date
2012
Author(s)
Chang, Yuan-Shu
Abstract
The institute of Earth Sciences, Academia Sinica (IESAS) has deployed eleven Sacks-Evertson-type borehole strainmeters in eastern Taiwan since 2002. The high sensitivity borehole strainmeter enable us to detect crustal strain in the scale of 10-12. Due to such high precision, the recorded strain is affected by environmental disturbances as well. In order to extract tectonic signals from borehole strainmeter data, it is of importance to clarify the influences of environmental factors on borehole strainmeter records. This study analyzed the datasets recorded by Sacks-Evertson borehole strainmeters installed in the central Coastal Range of eastern Taiwan. We first remove the effects of hole relaxation and grout-curing as well as strain steps due to valve resets and data gaps in the raw data using two exponential functions and a linear rate. Since it has been well known that atmospheric pressure would provide a significant influence on the crustal strain, we accordingly compute the barometric pressure admittance using a linear function. The SPOTL program is then applied in calculating the theoretical tide to obtain tidal admittance. The data processing method for single-component and three-component strainmeters are similar, except that the three-component instrument requires a coupling matrix to convert strain records at three gauges into the engineering shear strain and area strain.
Our preliminary results show the contractional strain may be affected by strong precipitation. Although the contractional signals associated with heavy rainfall are often recorded in dilatometer and areal strain, the recorded shear strain in 3-component strainmeter does not show significant changes at the same time. This finding indicates the hydrological loading due to rainfall only affects the volumetric strain and presumably has little influence on shear strain given assumption of isotropic materials. If the contractional signals are generated by fault activities, both shear strain and volumetric strain should record significant signal changes in the same time; nevertheless, shear strain doesn’t show significant signal changes while volumetric strain and areal strain have permanent strain changes. Thus, we eliminate the possibility of tectonic-origin motions.
We further compare month-scale volumetric strain from borehole strainmeters with surface dilatation rate derived from GPS velocity, in particular in the dry season. It shows correlated signal changes, suggesting that the borehole strainmeter record in month-scale may be a good index to recognize the crustal strain when the impact from the hydrologic influence is insignificant.
Subjects
borehole strainmeter
strain changes
Longitudinal Valley
precipitation
barometric pressure
Type
thesis
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